1. Field of the Invention
The present invention relates to coumarin derivatives, and more particularly to coumarin derivatives that, are suitable for use as an organic luminescent material to emit light of a desired color and an electroluminescent device in which the coumarin derivatives are used.
2. Description of Related Art
Organic electroluminescent (EL) material is attracting attention as a substance suitable for flat panel displays because EL material emits light spontaneously, responds at high speeds, and is independent of viewing angles. These features aroused attention about organic electroluminescent materials as a constituent of EL devices. One advantage of organic electroluminescent materials is that they have desired optical properties dependent on their molecular structure. Therefore, producing electoluminescent materials emitting three primary colors (red, green, blue) is possible by changing the molecular structure of the organic electroluminescent materials.
For EL display panels, the organic EL, materials must have the proper chromaticity and sufficient luminance to satisfy basic demands of the EL display panel, and a host-guest doped system offers a ready means of achieving the basic demands. The host-guest doped system is based on the principle of host-guest energy transfer to effect the spectral shift from tris-(8-hydroxyquinolinato)aluminum (Alq) to the dopant molecules of the organic EL materials.
The preferred dopant of the organic EL materials providing green light emissions in the prior art is 3-(2xe2x80x2-benzothiazolyl)-7-diethylaminocoumarin represented by formula coumarin 6 (C-6) and 10-(2-benzothiazolyl)-1,1,7,7-tetrainethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one represented by formula coumarin-545T (C-545T). Formula C-6 and C-545T have the following molecular structure: 
These dopants of organic EL materials are fluorescent dyes in an electroluminescent device and have molecules generally with high photoluminescent (PL) quantum yield in dilute solution and high green EL efficiencies when doped at an appropriate concentration level in an EL device using Alq as the host emitter. However, C-6 and C-545T are sensitive to concentration quenching. For example, C-545T drops luminant efficiency when the concentration level is below 1% and is very unstable for passive-matrix display manufacturing.
The present invention has arisen to mitigate or obviate the disadvantages of the conventional organic EL materials.
A first objective of the present invention is to provide coumarin derivatives that are more resistive to concentration quenching and do not significantly change the chromaticity characteristics.
A second objective of the present invention is to provide an electroluminescent (EL) device comprising at least one organic luminescent layer containing the coumarin derivatives having excellent luminant efficiency.